1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
In recent years, photolithography has made rapid progress to finer fabrication with finer semiconductor processes. In the next generation, microfabrication of 100 nm to 70 nm, and further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics.
There are three kinds of EUV light sources, namely, an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target, a DPP (discharge produced plasma) light source using plasma generated by electric discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has the advantages that extremely high intensity near black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, that an extremely large collection solid angle of 2π steradian can be ensured because of a point light source having substantially an isotropic angle distribution and no structure surrounding the light source such as electrodes, and so on. Therefore, the LPP light source is thought to be predominant as a light source for EUV lithography requiring power of several tens of watts or more.
Here, a principle of generating EUV light with the LPP system will be explained. When a laser beam is applied to a target material supplied into a vacuum chamber, the target material is excited and plasmarized. Various wavelength components including EUV light are radiated from the plasma. Then, the EUV light is reflected and collected by using an EUV collector mirror that selectively reflects a desired wavelength component (e.g., a component having a wavelength of 13.5 nm), and outputted to an exposure unit. For this purpose, a multilayer film in which thin films of molybdenum (Mo) and thin films of silicon (Si) are alternately stacked (Mo/Si multilayer film), for example, is formed on the reflecting surface of the EUV collector mirror.
In the LPP EUV light source apparatus, the influence by neutral particles and ions emitted from plasma is problematic especially when a solid target is used. Since the EUV collector mirror is located near the plasma, the neutral particles emitted from the plasma are deposited on the reflecting surface of the EUV collector mirror and reduce the reflectance of the mirror. On the other hand, the ions emitted from the plasma wears away the multilayer film formed on the reflecting surface of the EUV collector mirror (the shaving is also defined as “sputtering” in the present application). The scattered materials from the plasma including neutral particles and ions, and the remains of the target materials are called debris.
In order to maintain the high reflectance, the high flatness of about 0.2 nm (rms), for example, is required for the EUV collector mirror, and thus the mirror is very expensive. Accordingly, longer life of the EUV collector mirror is desired in view of reduction in operation costs of the exposure equipment, reduction in maintenance time, and so on. The mirror life in the EUV light source apparatus for exposure is defined as a period until the reflectance decreases 10%, for example, and at least one-year life is required.
In order to fulfill this requirement, International Publication WO 02/46839 A2 discloses special liquid droplet targets that are irradiated by a high power laser and are plasmarized to form a point source of EUV, XUV or X-ray. As various types of liquid droplet targets, solutions of metallic chloride, metallic bromide, and so on, or nano-sized particles (e.g., aluminum, bismuth, or the like) in solutions (e.g., water or oil) having a melting temperature lower than the melting temperature of some of the constituent metals are used. Since the target is in the form of droplets, the sufficient distance from the nozzle supplying the target to the plasma can be secured. Further, by restricting the mass of the target, the debris can be suppressed.
However, when the mass of the target is restricted to the degree that no debris is produced at all, various technical problems may occur and the apparatus may be complicated. For example, clogging occurs in the nozzle for supplying the target, and the EUV conversion efficiency (CE) decreases according to the reduction in target size. On the other hand, for optimization of the target size and density in order to improve the EUV conversion efficiency, it is necessary to expand the droplets by using pre-pulse laser.
Further, International Publication WO 2004/092693 A2 discloses a method and apparatus for debris removal from a reflecting surface of an EUV collector mirror in an EUV light source. Specifically, in FIGS. 2A and 2B of WO 2004/092693 A2, a debris shield including plural thin plates that define radially extending optical paths is shown. By locating the debris shield between a point light source formed at the plasma center and the EUV collector mirror, the debris deposited on the reflecting surface of the EUV collector mirror can be reduced.
However, since the debris shield is exposed to the plasma, the thin plates of the debris shield is worn away by fast ions and debris is produced, and the produced debris may be deposited on the reflecting surface of the EUV collector mirror. In this case, the debris shield itself becomes a debris source.
Furthermore, US Patent Application Publication US 2005/0279946 A1 discloses a system for protecting an internal EUV light source component from ions generated at a plasma formation site. In one aspect, the system may comprise a plurality of foil plates and equipment for generating a magnetic field to deflect ions into a surface of one of the foil plates. In another aspect, an electrostatic grid may be positioned for interaction with ions to reduce ion energy. However, the method of guiding debris by using a magnetic field or electric field is effective for ions, but not effective for neutral particles.
When a metallic film is deposited on the reflecting surface of the EUV collector mirror, EUV light is absorbed while making a round trip through the metallic film. Accordingly, when the optical transmittance of the metallic film becomes about 95%, the reflectance of the EUV collector mirror becomes about 90%. In order to hold the reduction in reflectance of the EUV collector mirror within 10% with respect to EUV light having a wavelength of 13.5 nm, the acceptable value of the amount of deposition (thickness) of the metallic film on the reflecting surface of the EUV collector mirror is a very little value such as about 0.75 nm for tin (Sn) and about 5 nm for lithium (Li).
Therefore, the achievement of one-year mirror life is considered to be very difficult in an EUV light source apparatus for exposure having output of about 115 W to 180 W at the focusing point only by restricting the mass of the target or providing a debris shield.